1. Field of the Invention
This invention relates to the determination of the location of the bottom of a hole drilled in the earth or some other downhole point in the wellbore. It relates especially to generation of a seismic signal at the drill bit (or other selected point along the drill string) and detecting the transmitted signals at the surface of the earth so that the horizontal and vertical position of the downhole point can be determined. It also relates especially to a novel apparatus for initiating the downhole seismic signal.
2. Prior Art
Various methods have been used to determine the location of the bottom of a borehole. For example, commercially available inclination and direction surveys can be run and an accurate determination made of the course of the borehole. One particular class of wells where it is especially important to follow closely the path of the bit during drilling is those that are purposely deviated to penetrate the producing stratum at a specific distance and direction of offset from the wellhead. It is to be remembered that when a borehole is drilled in the earth, it is almost never perfectly vertical. Its path frequently resembles an elongated corkscrew, and sometimes the deflection is much more in one direction than in another. This is the reason why it is both difficult and important to know the location of the bottom of the borehole as it is being drilled. In the prior art method of using the well survey, drilling must be stopped. Perhaps the closest prior art is U.S. Pat. No. Re 18,565, E. E. Rosaire, issued Aug. 9, 1932, for "Method of Determining the Straightness of Drill Holes in the Earth." In the Rosaire patent, the exact time of detonation of an explosive charge must be known; however, that is not a requirement in the present application.
Another patent of interest is U.S. Pat. No. 2,933,144, J. G. Scott, et al., issued Apr. 19, 1960. However, that patentee does not use his seismic signals which were generated at the drill bit in order to determine the horizontal location of it, such as is done in the present application.